In most vertebrates studied, including humans, data demonstrate that the future germ cells arise outside of the gonad, migrate to and colonize the sex organs, and these differentiate into ova or spermatozoa. One of the objectives of the proposed research is to further explore some of the factors involved in this directed migration of germ cells. Marked germ cells will be transferred from the early gonads of larvae to various sites (homotypic and heterotypic) of sterile host embryos to determine whether the germ cells retain the ability to migrate to the germinal ridges. Another objective will be to explore some of the factors involved in the differentiation of germ cells in the gonads. Triploid germ cells will be implanted into sterile diploid host embryos and in the reciprocal direction to determine whether it is the germ cells per se or the somatic gonadal environment which determines the direction of sex differentiation in certain heteroploid individuals. Finally, we will follow up a lead indicating that somehow originally sterile early gonads acquire limited numbers of germ cells during later development. A detailed cytological analysis will be done to determine the origin and possible fate of these germ cells. BIBLIOGRAPHIC REFERENCE: Subtelny, S., and L. Carrethers (1975). Interspecific germ cell migration in anurans. J. Cell Biol. 67: 421a.